1. Field of the Invention
The present invention relates to a buckle apparatus employed in a seatbelt system designed to protect an occupant of a vehicle in an emergency situation of the vehicle.
2. Description of the Related Art
A typical conventional buckle apparatus for use in a seatbelt system is arranged such that an occupant can be fastened by an occupant restraining webbing by engaging a tongue plate attached to an end or intermediate portion of the webbing with the buckle apparatus.
There are various types of buckle apparatus. One of them is a latch type buckle apparatus in which, as shown in FIG. 5, a tongue plate 12 is engaged with a lock member 10 which has its two ends respectively supported by members 14 which constitute the apparatus body. This latch type buckle apparatus has a large tongue plate holding strength and hence extremely high engagement reliability. In this buckle apparatus, disengagement of the tongue plate 12 is effected by sliding the lock member 10 upwardly as viewed in FIG. 5.
In this type of buckle apparatus, the tongue plate disengaging force, that is, the force F1 required to slide the lock member 10 when loaded, is represented by the following formula (1): EQU F1=(.mu.1+.mu.2)W.apprxeq.2 .mu.W (1)
where
W: webbing tension (load) PA1 .mu.1: coefficient of friction between the lock member 10 and the members 14 PA1 .mu.2: coefficient of friction between the lock member 10 and the tongue plate 12 PA1 (it is assumed that .mu.1=.mu.2=.mu.)
Accordingly, although the buckle apparatus of the type described above has a large tongue plate holding strength and hence extremely high engagement reliability, it also involves large frictional force, which leads to the disadvantage that a large force is required to disengage the tongue plate.